A Decreasing the power density of a laser

Hello! I have an issue with the power density of a 488 nm continuous wave laser. I'm using this laser to excite my sample. The problem is that even at low power, the laser is causing the samples to form a burn deposit on the quartz sample holder. Between the laser source and the sample, there are a series of dialectric mirrors, pinholes, and a focussing lens. What are the ways in which power density can be decreased at the focal point?

Thanks for the reply, the focussing lenses have knobs to adjust the focus in the x,y, and z direction. Do you think it matters which direction is defocus? Also, does it matter where in the focussing lens the laser beam hits? Thanks a lot.

Hello! I have an issue with the power density of a 488 nm continuous wave laser. I'm using this laser to excite my sample. The problem is that even at low power, the laser is causing the samples to form a burn deposit on the quartz sample holder. Between the laser source and the sample, there are a series of dialectric mirrors, pinholes, and a focussing lens. What are the ways in which power density can be decreased at the focal point?

Thanks for the reply, the focussing lenses have knobs to adjust the focus in the x,y, and z direction. Do you think it matters which direction is defocus? Also, does it matter where in the focussing lens the laser beam hits? Thanks a lot.

I don't know how your x, y and z directions are defined, but the relevant one is the optical axis along the laser beam. Let's call that the z-direction. I'm not sure what you mean by changing the focus in the x- and y-direction. Anyhow. the focusing lens is presumably bringing the laser beam to tight beam waist, thus concentrating its power in a small volume. If you move the focusing lens either toward or away from the laser, the beam waist at the sample will expand, thus lessening the power density.

The laser beam should go through the center of the focusing lens, otherwise it will be displaced in the x- or y-direction.

I don't know how your x, y and z directions are defined, but the relevant one is the optical axis along the laser beam. Let's call that the z-direction. I'm not sure what you mean by changing the focus in the x- and y-direction. Anyhow. the focusing lens is presumably bringing the laser beam to tight beam waist, thus concentrating its power in a small volume. If you move the focusing lens either toward or away from the laser, the beam waist at the sample will expand, thus lessening the power density.

The laser beam should go through the center of the focusing lens, otherwise it will be displaced in the x- or y-direction.

This might be naive on my part, but what about a beam splitter. A simple splitter between the laser and the beginning of the optics would reduce the power by 50% and might not require any adjustment to the optics. And I think there are some splitters that allow for adjusting the split ratio.

This might be naive on my part, but what about a beam splitter. A simple splitter between the laser and the beginning of the optics would reduce the power by 50% and might not require any adjustment to the optics. And I think there are some splitters that allow for adjusting the split ratio.

If interference effects are not deadly to you just add a bunch of microscope slides (with air spaces) somewhere in collimated laser path at some angle (approx. 45 degree). Each reflection will attenuate about 10 % of light if the incidence is s-polarized light (simply just rotate the slide and check for biggest reflection). Avoid direct back reflection as it will make laser unstable! Its cheap and easy.

If interference effects are not deadly to you just add a bunch of microscope slides (with air spaces) somewhere in collimated laser path at some angle (approx. 45 degree). Each reflection will attenuate about 10 % of light if the incidence is s-polarized light (simply just rotate the slide and check for biggest reflection). Avoid direct back reflection as it will make laser unstable! Its cheap and easy.

1 W is a very powerful laser. Please be careful of stray reflections (off the pinhole diaphram or ND filter, for instance), and use proper lab safety procedures (no public access to room when laser is running, warning signs, laser-filtering safety goggles for you and other lab personnel, etc.). Also be careful of attenuating filters. If they are placed in a position where the beam is focused, they can crack due to heating from the absorption. It is common to defocus the beam, attenuate it, and then refocus.